1. Field of the Invention
The present invention relates to a bobbin apparatus for holding a field coil of a brushless alternator and particularly to a joining construction between a yoke portion and a plate constituting the bobbin apparatus.
2. Description of the Related Art
Conventional bobbin apparatuses for holding a field coil of a brushless alternator are provided with: a yoke portion composed of a thick cylindrical thick portion and a thin cylindrical thin portion disposed so as to protrude axially from a minimum radius portion of this thick portion; a plate composed of a cylindrical portion and a disk-shaped flange portion disposed so as to extend radially outward from one end of this cylindrical portion; and a bobbin composed of a cylindrical portion and a pair of disk-shaped flange portions disposed so as to extend radially outward from two ends of this cylindrical portion. The cylindrical portion of the bobbin is mounted so as to be fitted over the cylindrical portion of the plate, the cylindrical portion of the plate is mounted so as to be fitted over the thin portion of the yoke portion, and the cylindrical portion of the plate and the thin portion are integrated by welding through welding apertures disposed through the cylindrical portion of the bobbin. (See Patent Literature 1, for example.)
Patent Literature 1: Japanese Patent Laid-Open No. HEI 11-55922 (Gazette, FIG. 21).
Generally, the smaller the winding diameter of the field coil, the more field coil material that can be saved because the length of the conducting wire can be shortened even with an equal number of winds, thereby enabling reductions in cost.
In conventional bobbin apparatuses, because the cylindrical portion of the plate mounted so as to be fitted over the thin portions of the yoke portion is integrated by welding, the inside diameter of the bobbin mounted to the cylindrical portion of the plate is increased by an amount proportionate to the thickness of the thin portion of the yoke portion. As a result, because the winding diameter of the field coil is larger, increasing the length of the conducting wire, one disadvantage has been that the amount of field coil material is increased, preventing reductions in cost. Furthermore, because a yoke portion having a thin portion is prepared from a round bar by turning, other disadvantages have been that the disposal rate of material is large and manufacturing time is lengthy, thereby preventing reductions in cost.
Because the cylindrical portion of the plate and the thin portion of the yoke portions are welded through welding apertures disposed through the cylindrical portion of the bobbin, another disadvantage has been that if there are changes in position or increases in the number of welds accompanying changes in the welding specification, it is necessary to prepare a bobbin in which the position and number of welding apertures are modified, reducing the degree of design freedom and also preventing reductions in cost.
Now, it is also conceivable that the thin portion could be omitted to reduce the inside diameter of the bobbin, and a second axial end surface of the cylindrical portion of the plate could be abutted to a first end surface of the yoke portion and integrated by welding. However, because the plates are then supported by the yoke portions using a cantilever construction, a first end of the plates that is farthest away from the portion fixed to the yoke portion becomes a free end, giving rise to new problems such as the plate vibrating significantly when mounted to an alternator.